1. Field of the Invention
The present invention relates to porous silicon, more particularly to light-emitting porous silicon in a lightemitting diode.
2. Prior Art
Photoluminescent porous silicon is a material that has been known for nearly forty years. It is usually formed by electrochemical etching in a hydrogen fluoride solution under an anodic current. Porous silicon has been used in microelectronics, especially in the silicon-on-insulator technology due to its ability to be a good insulator after oxidation. Canham first reported in Silicon Quantum Wire Array Fabricated by Electrochemical and Chemical Dissolution of Wafers, Appl. Phys. Lett., 57, pp. 1046-1048 (1990) that when porous silicon is further etched in hydrogen fluoride for hours after preparation, it emits bright red light under illumination with blue or UV light.
Electroluminescence (EL) was later observed in porous silicon in solution during oxidation (Halimaoui et al., Electroluminsa in the Visible Range During Anadic Oxidation of Porous Silicon Films, Appl. Phys. Lett., 59, pp. 304-306 (1991)) and then in a solid state device (Richter et al., Current-Induced Light Emission from a Porous Silicon Device, IEEE Electron Device Lett., 12, pp. 691-692 (1991)).
Solid state devices are presently commercially important in the electronics industry. A typical light-emitting porous silicon (LEPSi) light-emitting device (LED) includes a transparent or semi-transparent contact formed of gold or other conducting material and a 1-10 micrometer thick LEPSi layer on a crystalline silicon (c-Si) substrate. The c-Si substrate is doped with a controlled amount of impurities to form either a p-type or n-type conductor. In a p-type conductor, conduction results from movement of "holes" (absent electrons) through the material. In a n-type conductor, movement of electrons causes conduction. Threshold conditions for EL have been reported to be a voltage of at least 10 volts and a current density of at least 10 mA/cm.sup.2. Unfortunately these devices exhibit low EL external quantum efficiency (.ltoreq.0.01%) and quickly degrade irreversibly.
The low efficiency and irreversible degradation of LEPSi LEDs previously rendered them unacceptable for semiconductor device applications. In particular, the stability of most LEPSi LEDs is poor. To date, LEPSi LEDs degrade within minutes when placed in air and after a few hours when placed in a moderate vacuum. The Si--H bonds that passivate the silicon nanocrystal surfaces are very fragile and can be easily broken by exposure to light, ambient air, moderate temperatures and large electric fields. Because the temperature of the LEPSi layer in a device driven well above the EL threshold can reach about 100.degree. C. and the local electric field can be well in excess of the typical macroscopic field of10.sup.4 V/cm, the Si--H bonds can be broken relatively easily and rapid degradation follows.
Accordingly, a need remains for a LEPSi device with improved stability.